How does a mismatch between the price charged and actual equilibrium or market-clearing price tend to generate a surplus or a shortage?

The market outcomes of a good: the price of the good and the quantity of the good in the market is determined by the forces of demand and supply. The market attains equilibrium at the point where the quantity demanded of the good is equal to the quantity supplied of the good. At this point the price at which quantity is demanded by the consumers and the price at which quantity is supplied by the sellers is also equal.

If the sellers then, start charging price which is higher than the equilibrium price of the market, the quantity supplied at a higher price will become more than the quantity demanded at that price. The supply being greater than demand,it will create surplus in the market, and will exert a downward pressure on the price of the good. The price of the good will thus tend to fall till the point where the quantity demanded becomes equal to quantity supplied.

On the other hand, if the sellers sell a price which is less than the equilibrium price, the quantity demanded at this lower price will be greater than the quantity supplied at this price. This will create a shortage of goods in the market and will put an upward pressure in the price. As a result, the price of the good will tend to increase till the point where the quantity demanded becomes equal to quantity supplied.